This invention relates generally to wind machines, and in particular to so-called horizontal-shaft wind machines.
A wind machine is a device for extracting energy from the wind. A typical horizontal axis wind machine consists of a horizontally pivoting platform to the top of a mast. A rotor assembly consisting of a plurality of sails or blades is attached to a horizontal shaft, which is supported for rotation on top of the platform. The horizontal shaft is joined to a power transmission system which is coupled to a work-performing device, for example an electrical generator or pushrod for pumping water. Wind machines can generally be divided into two categories: Lift-type and drag-type. Lift-type wind machines use slender airfoils similar to airplane propellers, which create lift as the wind passes over the airfoils to rotate the wind machine rotor assembly. As with aircraft propellers, the pitch of the lift-type airfoils must be matched to the windspeed for maximum efficiency and the pitch typically varies from the root of the airfoil to the tip in order to compensate for the different path velocity of the airfoil along the leading edge. Drag-type wind machines use relatively wide sails with large surface areas, which act to slow the wind striking the sails and convert a portion of the kinetic energy of the wind into rotary motion of the wind machine rotor. Drag-type wind machines typically include a rudder that extends from the rear of the wind machine head for aligning the wind machine so that the rotor always faces the wind.
Conventional wind machines, be they lift-type or drag-type typically comprise a single set of blades or a single rotor rotating about a horizontal shaft. Mathematically, the aerodynamic efficiency yielded by a single set of blades or by a single rotor cannot exceed 59.6% (See Betz, A. “Wind-Energie and Ihre Ausnutzun durch Windmuelen,” van den Hoeck & Ruprech, Goettingen, 1926). In practice, the output of typical wind machines is substantially below 59.6%.
Various apparatus have been proposed to increase the efficiency of wind machines including use of multiple rotors and/or a shroud, which increases the effective swept area of the rotors and, therefore, the overall efficiency of the system.
U.S. Pat. No. 6,452,287 discloses a horizontal axis wind machine in which a single rotor is surrounded by a shroud that is supported by the mast with the shroud in a position forward of the rotor. Close clearance between the shroud and the tips of the rotors reduces the generation of rotor tip vortices and the concomitant loss of efficiency. A commercially available wind machine known as the Wind Tamer™ from Arista Power, Inc. incorporates single rotor with a large diffuser supported by the mast with the diffuser/shroud behind the rotor. Another commercially available wind machine from Ogin, Inc. known as the Flo Design Wind Turbine incorporates a single rotor with a lobed nozzle extending in front of the rotor and a separate diffuser that is spaced radially apart from and offset to the rear of the nozzle. Both the nozzle and the diffuser are supported by the mast. None of the prior art, however, discloses or suggests a shrouded rotor in which the shroud is attached to the rotor shaft, or otherwise rotates with the rotor as it spins and none of the prior art discloses or suggests a multiple-rotor wind machine with multiple individual shrouds.